1. Field of the Invention
The present invention relates generally to a process for the recovery of usable economically valuable products, including a relatively pure iron or direct reduced iron product feedstock and, optionally, an iron oxide and an iron-carbon residual, from industrial waste streams typically comprising zinc compounds and iron compounds. The process comprises the general steps of an ammonium chloride leach and an elevated temperature roast, along with specific steps for recovering the desired products. The present invention also relates generally to an enhanced recycling process which utilizes iron-rich materials produced by the invention as a feedstock ultimately to a steel mill.
The specific improvement of the present invention is an additional process for the removal of calcium impurities remaining after the ammonium chloride leach. Due to the continuous nature of the process, calcium impurities may build up, resulting in lower efficiency. The use of a secondary ammonium salt, different from ammonium chloride, helps alleviate this calcium impurity build up, maintaining efficiency.
2. Prior Art
Industrial waste streams typically contain components which have economic value if they can be recovered in an economic fashion. For example, U.S. Pat. No. 3,849,121 to Burrows, now expired but which was assigned to a principal of the assignee of the present invention, discloses a method for the selective recovery of zinc oxide from industrial waste. The Burrows method comprises leaching a waste material with an ammonium chloride solution at elevated temperatures, separating iron from solution, treating the solution with zinc metal and cooling the solution to precipitate zinc oxide. The Burrows patent discloses a method to take EAF dust which is mainly a mixture of iron and zinc oxides and, in a series of steps, to separate out and discard the iron oxides and waste metals, so that the resulting zinc-compound-rich solution can be further treated to recover the zinc compounds.
Waste metal process dust typically has varying amounts of other components, in various forms, such as calcium, contained in the dust. The Burrows patent does not teach the treatment or recovery of any values from the discarded iron oxide containing precipitates, and does not discuss any method of relieving the build up of impurities, such as calcium compounds, which occurs during continuous operation of the process.
U.S. Pat. No. 4,071,357 to Peters discloses a method for recovering metal values which includes a steam distillation step and a calcining step to precipitate zinc carbonate and to convert the zinc carbonate to zinc oxide, respectively. Peters further discloses the use of a solution containing approximately equal amounts of ammonia and carbon to leach the flue dust at room temperature, resulting in the extraction of only about half of the zinc in the dust, almost 7% of the iron, less than 5% of the lead, and less than half of the cadmium. However, Peters does not disclose a method for further treating the removed components not containing zinc compounds, nor of removing impurities, such as calcium compounds, which build up during the process.
As can be seen, there exists a need for a method which will allow the continuous treatment of exhausts and fumes from reduction furnaces or the like to recover values in a manner which relieves or avoids the build up of calcium impurities which can affect the efficiency of the process. This need is addressed by the present invention.